Metal antioxidants for fluoroesters



United States Patent 3,290,242 METAL ANTIOXIDANTS FOR FLUOROESTERSHarold Ravner, Accokeek, and Carter 0. Timmons, Oxon to the UnitedStates of America as re resented by the Secretary of the Navy v No lrawing. Filed Apr. 18, 1963, Ser. No. 274,064 4 Claims. (Cl. 252-26)Hill, Md., assignors to improvement in the oxidation stability offluoroesters at high temperatures. I

Developments in military and rndustrial equipment have presented anincreasing demand for lubricants and hydraulic fluids which can meetsevere operational requirements. Among such requirements is oxidationstability at high temperatures.

Fluid neutral esters of omega-hydroperfluoroalkylcarbinols and saturatedaliphatic hydrocarbon polycarboxylic acids have been heretoforesuggested as a source of potential high temperature lubricants andhydraullc fluids. As a class these esters lack the necessary oxidationstability which would allow their use as lubricants or as hydraulicfluids at temperatures in the range of about 400 to 500 F. p

It is an object of the present invention to improve the oxidationstability of fluoroesters of the aforesaid class whereby to provide newhigh temperature lubricants and hydraulic fluids.

We have found certain metals to be antioxidants which impart anexceptional degree of oxidation stability at elevated temperatures inthe range of about 400 to 500 F. to fluid neutral esters ofomegahydroperfluorolkylcarbinols which have from 5 to 11 carbon atomsand saturated aliphatic hydrocarbon ditriand tetracarboxylic acids whichhave from 3 to 9 carbon atoms in the hydrocarbon groups.

The metals which we have found to be antioxidants for the fluoroestersof the aforedefined group are barium, tin and zinc. The thresholdtemperature for antioxidant activity of the metals in the fluoroestersis about 400 F. Below this temperature, the fluoroesters are inthemselves oxidation stable. The metals are effective as antioxidantseven in small concentrations, as low as on the order of from about 0.05to 0.5% by weight based on the weight of the fluoroesters, to impart ahigh order of oxidation stability to the fluoroesters at temperatures inthe range of from about 400 to 500 F. The antioxidant metal may bebrought into contact with the fluoroesters not only in particle form butalso in the form of plates or other larger shapes or configurations asfound suitable. The surface of the parts of engines or of othermechanisms with which the fluoroesters come into contact, such as oillines, may consist of the antioxidant metal or a by-pass system maycontain the antioxidant metal in the form of a wire mesh or as aconfined mass of grandules through which the fluoroester flows incontact before returning to areas of lubrications.

The mechanism whereby the metals function as antioxidants in thefluoroesters is not known, but is believed to involve the formation ofmetal ions in the fluoroesters.

The fluid neutral fluoroesters with which the present invention isconcerned may be prepared in known way as as described in Ind. Eng.Chem. 48, 445 (1956) from the omega-hydroperfluoroalkylcarbinols of thegeneral formula:

"ice

wherein n is an integer from 2 to 5 and aliphatic polycarboxylic acidsof the general formula:

R(COOH) wherein n is an integer from 2 to 4 and R is a saturatedaliphatic hydrocarbon group having from 3 to 9 carbon atoms. Thus, theneutral fluoroesters may be derived by the complete esterification of,for example, glutaric, adipic, pimelic, suberic, azelaic, sebacic andundecanedioc acids; methyl succinic, S-methylglutaric, 1,4-dimethyladipic, 2,2-dimethyladipic, l-methyl-4-ethyl adipic acids, etc.;camphoric acid, tricarballylic acid, butane-1,2,4-tricarboxylic acid,butane 1,1,4-tricarboxylic acid, etc., andbutane-1,2,4,4-tetracarboxylic acid, butane-l,l,2,3-tetracarboxylicacid, butane-l,1,2,3-tetracarboxylic acid, etc., with fluoroalcohols ofthe above general formula which are 1H,1H,'5H-octafluoro-1-pentanol,1H,lH,7H-dodecafluoro l-pheptanol, 1H,lH,9H-hexadecafluoro-l-nonanol and1H,lI-I,llH-eicosafluoro-l-undecanol. Illustrative of these fluidneutral fluoroesters are those described in Ind. Eng. Chem. 48, 445,above, for example, bis (1H,lH,5I-I- octafiuoropentyl)- and his(1H,1H,7H-dodecafluoroheptyl) 3-methylglutarates, bis(1H,1H,5H-octafiuoropentyl) adipate and sebacate, tri(1H,1H,5H-octafluoropentyl) tricarballylate, etc.

, temperatures.

The antioxidant activity of the metals for the fluoroesters may beevaluated in terms of increase in the viscosity and rise in theneutralization number of the fluoroesters and the amount of fluorideevolved from the fluoroesters. The lower the value for these factors,the greater in general the oxidation stability imparted to thefluoroesters by the metals at a given temperature in the aforesaidrange. Depending upon the conditions of service use, all or less thanall of these factors may govern in the selection of the fluoroesters onthe basis of oxidation stability. Thus, where the acid developed in thefluoroester is not a corrosion problem, either by virtue of the acid initself being non-corrosive or the metal parts with which the fluoroesterand developed acid come into contact are acid resistant, rise in theneutralization number can be of minor significance. Similarly, evolvedfluoride also can be of minor significance where means are provided forventing of the evolved fluoride from an engine or other system in whichthe fluoroesters are employed.

Increase in viscosity and rise of the neutralization number of thefluoroesters and the amount of fluoride evolved therefrom on heating thefluoroesters to temperature in the aforesaid range in the presence ofthe metals may be deter-mined by use of the dynamic oxidation testprocedure and apparatus described in Ind. Eng. Chem. 39, 491 (1947). Inthis test, the samples of the fluoroesters with the metal specimenimmersed therein are contained in cylindrical cells made of borosilicateglass. The metal specimens, which may be in the form of strips, are usedin clean, degreased condition. The control is a like volume of thefluoroester in a cell but without a metal therein. Each of the cells isequipped with a watercooled condenser fitted to the top thereof and hasa rubber hose connected from the condenser to a separate water trap.Sealed to the upper wall of each of the cells is a means for deliveringbubbled air into the bottom of the fluoroester samples in the cells. Thecells containing the fluoroester samples and metal specimens and thecell with the control sample are placed in snugly fitting holesi0.5 F.Clean dry air, free from CO is bubbled through the heated fluoroestersamples in the cells at a predetermined volume and rate. The efiluentgases from each of the cells which have passed through the individualassociated condenser are led into the water trap provided therefor andthe fluoride content of the trap liquid determined by the leadchloro-fluoride method. The amount of evolved fluoride is reported asmg. of fluorine per gram of the fluoroester. The viscosity incentistokes at 100 F. and the neutralization number of the fluoroestersare determined before and after the test. The neutralization number isthe number of milligrams of potassium hydroxide required to neutralizethe fluoroester.

Indicative of the oxidation stability which can be imparted by the metalantioxidants of the invention to the fluoroesters of the aforesaid groupat temperatures in the range of 400 to 500 F. are the results obtainedon commercial (Du Pont Company) fluid mixed camphorate esters from thecomplete esterification with camphoric acid of mixed C C and Cfluoroalcohols of the formula H(CF CF CH OH in which the ratio of thefluoroalcohols was approximately 0.5 mole of the C fluoroalcohol, 1 moleof the C fluoroalcohol and 0.5 mole of the C fluoroalcohol. The mixedcamphorate fluoroesters had a microboiling point of 351 C. The testswere conducted at 482 F. for 72 hours using 20 ml. samples of the mixedfluoroesters and metal specimens in the fluoroester samples whichgenerally had surface areas of 10 square centimeters. The air wasbubbled into the fluoroester samples in the cells at the rate of 1 cc./gram fluoroester sample/minute. The results of the tests with theseveral metals as antioxidants are set forth in the following table.

DYNAMIC OXIDATION TEST Viscosity Neutralization Fluoride Metals Increaseat N 0. Increase Evolved 100 F. mg. F./gm. (percent) Gelled 36 5. 3 2. 0Nil 3. 2 0. 45 38 11.0 2. 2

' and not in limitation except as may be defined in the appended claims.

What is claimed is:

1. A composition comprising a fluid neutral ester of a fluoroalcohol ofthe general formula:

H (CF CF CH OH wherein n is an integer from 2 to 5 and an aliphaticpolycarboxylic acid of the general formula:

. wherein n is an integer from 2 to 4 and R is a saturated aliphatichydrocarbon group having from 3 to 9 carbon atoms and a metal of thegroup consisting of barium, tin and zinc present therein in amountsuflicient to improve the oxidation stability of the fluid neutral esterat tem- J peratures in the range of from about 400 to 500 F.

2. A composition as defined in claim 1, wherein the metal is barium.

3. A composition as defined in claim 1, wherein the metal is tin.

4. A composition as defined in claim 1, wherein the fluid neutral esteris a camphorate of at least one fluoroalcohol of the general formula:

I wherein n is an integer from 2 to 5.

References Cited by the Examiner UNITED STATES PATENTS 4/1956 Reilf252-56 FOREIGN PATENTS 610,516 12/1960 Canada. 866,053 4/1961 GreatBritain.

OTHER REFERENCES Mardles: International Tin Research and Dev. Council,Tech. Publn., Series C #2, pp. 1 to 5 (1934).

Kalichevsky et 211.: Petroleum Refining With Chemicals (1956), ElsevierPub. Co., p. 590.

Gunderson et al.: Synthetic Lublicants, Reinhold Pub. Co. (1962), p.189.

DANIEL E. WYMAN, Primary Examiner.

W. H. CANNON, Assistant Examiner.

1. A COMPOSITION COMPRISING A FLUID NEUTRAL ESTER OF A FLUOROALCOHOL OFTHE GENERAL FORMULA: